1. What the date means
The date on a pack is a best-before date for shelf life, not an expiry date. A coin lithium cell keeps around 10 years, alkaline around 5 to 7, and a zinc-air hearing aid battery only 3.
2. Charge fades slowly
A stored battery loses a little capacity every month. It doesn’t go flat and then recover. It just gets weaker, so the device runs out sooner.
3. Check before you rely on it
A coin cell sitting at 3.0 volts is fine. Below about 2.8 volts, or below 1.2 volts in an AA, treat it as dead and replace it.
4. Buy what you need
Batteries kept for years are often thrown away unused. Buy a small pack, use them up, and replace them when the device says so.
5. Storage still counts
Heat and damp shorten shelf life. Keep spares in the pack they came in, somewhere cool and dry.